1. Field of the Invention
The present invention relates to an electrically conductive roll which is particularly favorably used as a charging roll or an image developing roll for an electrophotographic copying machine, printer or the like. This invention is also concerned with a method of producing such a conductive roll as described above.
2. Discussion of Related Art
An electrically conductive roll, such as a charging roll or an image developing roll used in an electrophotographic copying machine, facsimile machine or the like, has been desired to meet the following requirements: (1) the roll can be held in good contact with a photosensitive drum, (2) the photosensitive drum is not contaminated by a softener which oozes or comes out onto the outer surface of the roll, (3) the electric resistance of the roll is suitably controlled, and (4) the roll has relatively small tackiness and is less likely to stick to the photosensitive drum.
To fulfill all the above requirements, an electrically conductive roll as shown in FIG. 6 has been proposed by the assignee of the present application in laid-open Publication No. 2-311868 of unexamined Japanese Patent Application. As is apparent from FIG. 6, the conductive roll 42 has an electrically conductive shaft 44, and a base layer 46 which is formed on the outer circumferential surface of the shaft 44 and is made of an electrically conductive, elastic material. On the outer circumferential surface of the base layer 46, there are formed by coating a softener-preventing layer 48 (which will be described later), a resistance adjusting layer 50 formed of a particular semi-conductive rubber, and a protective layer 52 formed of a semi-conductive resin, in the order of description. The preventive layer 48 is formed of a suitable conductive resin which contains an electron-conductive material.
In the conductive roll 42 constructed as described above, the conductive elastic material for the base layer 46 contains a suitable softener which serves to lower the hardness of the base layer 46, and thus advantageously enhance contact between the roll 42 and a photosensitive drum (not shown). The softener-preventing layer 48 formed on the outer surface, of the base layer 46 serves to prevent the softener contained in the base layer 46 from oozing or migrating onto the outer surface of the roll 42, thereby to effectively avoid contamination of the photosensitive drum. The above-indicated resistance adjusting layer 50 is formed of a conductive material having a volume resistivity which is held in a suitable range such that the electrical resistance of the roll is desirably controlled within an appropriate range. Further, the protective layer 42 as the outermost layer of the roll 42 effectively prevents the roll 42 from sticking to the photosensitive drum which the roll 42 is to be pressed against and held in contact with. Thus, in the conductive roll 42 as disclosed in the above-identified publication, the four individual layers 46, 48, 50, 52 formed of different materials are stacked or laminated integrally into a tube formed on the outer circumferential surface of the shaft 44, so that the resulting roll 42 satisfactorily meets all the requirements (1) through (4) as described above.
As well known in the art, when a voltage is applied to a charging roll as one type of the conductive roll while the roll is being pressed against and held in contact with a photosensitive drum, the photosensitive drum vibrates due to force which acts between the charging roll and the photosensitive drum upon changes in the frequency of AC fields. When the charging roll is used in a high-speed copying machine, printer or the like which performs its copying or printing operation at a high speed, in particular, the vibration of the drum is increased so much as to cause undesirable noise. Therefore, the conductive roll, particularly the charging roll, is required to have an excellent vibration absorbing characteristic, as well as the above-described characteristics. The conductive roll having all the required characteristics is capable of absorbing the vibration of the photosensitive drum, and easily and effectively avoiding the noise.
To deal with the vibration which takes place between the conductive roll and the photosensitive roll and causes noises, the above-identified publication proposes that a relatively large amount of softener be contained in the base layer 46 to reduce the hardness of the layer 46, so as to effectively prevent the vibration as described above. However, the thus obtained conductive roll 42 inevitably suffers from an increased amount of the softener which oozes out of the roll 42 due to the large content of the softener in the base layer 46.
The oozing of the softener may be prevented to some extent by increasing the thickness of the softener-preventing layer 48 of the conductive roll 42. However, the increase in the thickness of the softener-preventing layer 48 causes other problems during the process of forming the layer 48 by coating, for example, sagging of a coating liquid which forms the softener-preventing layer 48. Thus, the conductive roll 42 as disclosed in the above publication still has some room for improvement in terms of its vibration absorbing characteristic.
The conductive roll 42 exhibits the above-described excellent characteristics due to provision of the softener-preventing layer 48, resistance adjusting layer 50 and protective layer 52 on the outer surface of the base layer 46 formed on the outer surface of the shaft 44. Since these three layers 48, 50, 52 are formed by coating on the base layer 46, the conductive roll 42 suffers from various problems in terms of its product capability or performance and the process of manufacturing the roll 42.
More specifically, in order to achieve the uniform thickness of each of the coating layers 48, 50, 52 of the conductive roll 42 in the circumferential direction thereof, the coating operation is effected while a base roll consisting of the shaft 44 and the base layer 46 is being supported so as to stand upright. During this coating operation, sagging of a coating liquid which gives each of the layers 48, 50, 52 inevitably takes place, whereby the thickness of each coating layer 48, 50, 52 is undesirably increased in the direction from one axial end thereof toward the other end. If the coating liquid sags excessively, a clearance or gap appears between the obtained conductive roll 42 and the photosensitive drum, resulting in uneven charging of the drum. Further, corona discharge may take place exclusively at the clearance between the roll 42 and the photosensitive drum, and a portion of the photosensitive drum which corresponds to the clearance may wear faster than the other portions in a long period of use, whereby the electrical resistance of the photosensitive drum is lowered. As a result, an abnormal discharge may take place upon application of a low voltage. In an extreme case, the image reproduced by the conductive roll has a low copy quality, that is, lines undesirably appear as a part of the reproduced image in the transverse direction of the copy sheet.
In producing this conductive roll 42, a solvent is used in the above-described coating operation for dissolving a suitable rubber material or resin material. The use of the solvent requires considerable concern for safety, and also causes bubbles and cissing or crawling to appear on the coating layers 48, 50, 52. To enhance yield of the conductive roll 42, the temperature, humidity, gas volume or quantity for drying the coating layers 48, 50, 52 need to be considerably accurately controlled. Moreover, there is a limit to the material used for each of the coating layers 48, 50, 52, which is selected depending upon a specific kind of the solvent used for dissolving the material.
Where a granular electron-conductive material is added to the material for any of the coating layers 48, 50, 52 of the conductive roll 42, the coating liquid including the conductive material must be constantly stirred during the coating operation, so as to reduce a variation of the electrical resistance in the coating layer 48, 50, 52. It necessitates bulky equipment which is capable of effecting the coating while stirring the coating liquid.
In the conductive roll 42 as disclosed in the above-identified publication, the resistance adjusting layer 50 is required to have a relatively large thickness of about 100-200 .mu.m, so as to assure an increased resistance to leak (leakage of electric current). Since such a resistance adjusting layer 50 cannot be formed by one coating cycle without causing sagging of the coating liquid as described above, the coating cycle must be repeated many times to gradually increase the thickness of the resistance adjusting layer 50. Further, the viscosity of the coating liquid must be also controlled so as to assure high uniformity in the thickness of the layer 50. These requirements eventually make the process of forming the three coating layers including the resistance adjusting layer 50 extremely cumbersome and low efficient.
Moreover, the conductive roll 42 has C-shaped or R-shaped axially opposite ends so as to avoid spark discharge at the axial end faces thereof. Therefore, the thickness of each of the coating layers 48, 50, 52 is locally reduced due to the surface tension of the coating liquid, at a point of inflection formed at the boundary of the axial end portions of the roll 42 and its axially middle portion. Accordingly, the resultant roll 42 inevitably includes a portion having a relatively small thickness even if the thickness of the resistance adjusting layer 50 is accurately controlled by the cumbersome process as described above. This results in high possibility of leakage of electric current due to discharge breakdown occurring at the small-thickness portion of the roll 42.